PDA

View Full Version : turning unwanted tapers



loose nut
11-24-2012, 09:50 PM
I was turning a spindle recently using 416 SS. If I made a cut with a good depth of cut (or even as low as .005" doc) and a fairly high speed (carbide inserts) I got a very nice finish but with a slight taper, about .0007 to .0012" over 6". Now if I take another cut at the same depth the taper is machined out but the finish sucks, unwanted lines, dig ins etc. I'm making an assumption that this is due to the tool springing up and down because of lack of loading when the cut is very light, so how do I get rid of this so I can take a light finishing cut.

lakeside53
11-24-2012, 09:55 PM
Light cut? how light? Your carbide insert will have a minimum DOC that may exceed your requirements... If so, move to HSS, or take a deeper cut to your final dimension. Are you supporting via the tail stock? If so and you have a taper consider moving the tail stock center. If you get good results at 0.005, make that your final cut and don't attempt a "spring cut" with carbide - it basically rubs, not cuts.

Alternatively... use emery to get rid of the taper. Easy to do and I'm guilty of it all the time ;)

loose nut
11-24-2012, 10:02 PM
I was using the tailstock and it is dead on, I don't move it.

Light cuts, 1 or 2 thou, enough to bring something to size, if necessary. Taking the deeper cut is what is giving me the problem. HSS might solve it.

firbikrhd1
11-24-2012, 10:18 PM
Stainless has a tendency to work harden when it is machined and that makes shallow cuts difficult. Rigidity of your setup is important for both finish and true turning end to end. If using a tailstock it must be set up properly to assure straight turning as lakeside53 said. If not using one and it is possible to do so that may help with deflection. Even so, the material can flex to some extent while being machined producing taper or smaller diameter at the center point that at the ends in the case of tailstock use, particularly in the case of a relatively dull edge on carbide as compared to HSS. Add the effects of work hardening and you will find it very difficult to remove tiny amounts on a final pass to any degree of accuracy and retain a good finish. When turning stainless a tool bit with a keener edge may help, HSS as suggested by lakeside53 can be sharpened to a keener edge than most carbides. The best method is to deduce how much is actually being removed during initial passes and make your final pass deep enough to take off the work hardened surface and arrive at your finished diameter at the same time.

lakeside53
11-24-2012, 10:20 PM
If you are using the tailstock and tool pressure is the culprit then the taper should be "hour glass" shaped. If you have a continous taper, your tail stock is off or it's moving.

In any case, very few carbide inserts will cut at 1-2 thou in 416.

Bob Fisher
11-24-2012, 10:46 PM
Very sharp HSS is the way to go on most HSM machines. I have some insert tooling, but seldom use them any longer. I did buy some HSS inserts at a NAMES show, and use those often. But unless you have a lot of power and speed, I think HSS will eliminate your problem. Bob.

Chuck K
11-24-2012, 11:03 PM
What the others said. You might try hss ground as a vertical sheer cutting bit. I have taken less than .001 doc with them and had a perfect surface finish. I haven't tried them on stainless though.

Chuck

darryl
11-25-2012, 02:26 AM
This might be a case where you could play with the cutter height, and in the case of HSS cutter, you could play with clearance and rake angles. There is probably an optimum here where the work neither springs away nor sucks in towards the cutter. That would let you vary the depth of cut a little without affecting the amount of material that comes off. You could dial in 5 thou and that's what comes off, then you could dial in 1 thou and that's what comes off.

I also like to dispense with the compound and mount the tooling more directly to the crosslide to lessen flex. Depends on the lathe and its parts of course, but it can be a factor.

oldtiffie
11-25-2012, 02:32 AM
Use a tool-post grinder with an aluminium oxide wheel - but see that the wheel doesn't "clog up" and keep it well "dressed".

Jaakko Fagerlund
11-25-2012, 04:47 AM
Or just forget the HSS and grind one of your carbide inserts to a sharp edge. Have done this many times, just requires a diamond wheel.

I see again people thinking that carbide can't be sharpened and that it is always "dull", but if that was the case there would be no carbide end mills.

big job
11-25-2012, 07:37 AM
And then some smaller machines like my 9A SB just dont like carbide, it wants to act
more like a parting tool. I know this sounds way out there, but, if you have a quick
change tool post/holder, stick a HSS twist drill in it. Its really nothing new, for years
twist drills were used with shapers. Use the fattest that will go in there keep it pretty
well choked up, and a small flat stock on the top to tighten it. Then you (turn) its
cutting edge to "dial it in". Then try it with a piece of 'junk' you will get the idea to
dial it in.

uncle pete
11-25-2012, 08:03 AM
One more thing to try? Raise the feed per rpm. Yeah I know it sounds counter intuitive for a finish cut, but sometimes upping the feed per rev gives you a better surface finish. Your carbide type, tip radius, depth of cut, etc. could or may be right on the verge of starting to chatter. At a micro level it may be already doing so. Increasing the feed loads the cutting tool better on the lighter depths of cut. It's worth a try anyway.

Pete

loose nut
11-25-2012, 10:30 AM
Use a tool-post grinder with an aluminium oxide wheel - but see that the wheel doesn't "clog up" and keep it well "dressed".

Good idea, if only I had one.

vpt
11-25-2012, 10:55 AM
Personally I use an assortment of files to finish off projects all the time. With some parts I will turn down to within .001-4 of the final finish and then file in the rest. One stroke of my bigger file will take SS down about .0005, the smaller files obviously less. A nice clean file can leave a near mirror finish if done right.

Mcgyver
11-25-2012, 03:45 PM
If you are using the tailstock and tool pressure is the culprit then the taper should be "hour glass" shaped. .

hmmmm, maybe. as you go on to note "or its moving". I think that is more the case. Even on a half decent lathe, set a tenths indicator up and lean on the barrel....he only has to be out .0003" to get that taper.

Are you using a rotating centre; newish Rohm, beater offshore? The centre through its bearings can also allow some movement.
Ultimate accuracy is best with a dead centre.

I to would go with very sharp hss, oil and slow speed as a dodge. I suppose the full professional answer would be 1) make the set up as rigid as possible, 2) pick your final desired DOC, 3) through some trial cuts adjust the tailstock such that there is no taper at that DOC.

It should be better than you are getting, but keep in mind eliminating taper to a small number tenths over a long length is both difficult and not often required.

vpt
11-25-2012, 03:54 PM
Maybe use a follow rest?

loose nut
11-25-2012, 05:21 PM
Maybe I'm expecting to much??

darryl
11-25-2012, 05:42 PM
Not expecting too much- you're blessed with a challenge, that of fine-tuning your expertise in machining :)

rohart
11-25-2012, 09:10 PM
I disagree with lakeside saying that if the taper is caused by tool pressure you'll get an hourglass shape. If the work bends away from the tool midway, you'll get a bobbin shape, wider in the middle, because the work bends away from the tool resuling in less removal, and a thicker final product.

Otherwise, I agree with all the above - HSS for finishing, or Jaako's suggestion of sharpening carbide with diamond.

But the idea of finding exactly the right DOC with HSS to remove the work-hardened layer but still get a finish sounds like the kind of work you'd if you were in a production environment. It doesn't seem to fit the home shop.

If you have a carbide you can sharpen to a larger radius, you might be able to give it enough to do with a higher feed but lower DOC, but that's a technique that I haven't tried yet.

I have a carbide insert that's very picky - lower the feed a little and the finish is terrible. Up the feed and it's all shiny again. But often the shiny is an illusion, and it needs HSS or paper to get a polished finish.

Of course, grinding is the answer, SS or not.

lakeside53
11-26-2012, 12:27 PM
lol.. you are right! ok, inverse hour-glass! Sorry, right idea but I wasn't thinking of the right word. Wonder if I got it right when I said my wife has an hour glass body? ;)

Kevinb71
11-26-2012, 02:33 PM
lakeside
I suggest you don't ask your wife that question? You may not "see" the answer for alwhile. LOL

loose nut
11-26-2012, 07:44 PM
A couple of the reply's mention SS work hardening but this is 416 SS, free cutting and usually machine very easily. Is work hardening still a problem with this type of steel???? I can get a very nice finish on it, it's just the slight taper that needs to go away. Grinding isn't an option, home shop without the equipment.

lakeside53
11-26-2012, 08:09 PM
Only a little (comparatively). if you tailstock isn't the problem, HSS will work fine. Grinding is an option - just use emery paper. You will be suprised how easily you can get the entire shaft polished and within a couple of 10ths.

firbikrhd1
11-26-2012, 09:37 PM
A couple of the reply's mention SS work hardening but this is 416 SS, free cutting and usually machine very easily. Is work hardening still a problem with this type of steel???? I can get a very nice finish on it, it's just the slight taper that needs to go away. Grinding isn't an option, home shop without the equipment.

I'm certainly no expert but information found at this link:
http://www.hazmetal.com/f/kutu/1236776379.pdf
suggests that all stainless, even free machining types, harden to some extent. According to the information provided 416 is at Martensitic stainless with medium work hardening properties (see page 8).